One way to mount a semiconductor chip (semiconductor element) on a wiring substrate is flip-chip mounting. Flip-chip mounting is one mounting method where a projecting electrode (bump) is provided for a surface of a semiconductor chip and this surface is directly connected to a wiring substrate. For protecting the connected semiconductor chip and wiring substrate and the bumps, the space between the semiconductor chip and the wiring substrate is filled with the sealing resin called an under-filling agent.
For the under-filling agent, epoxy resin is mainly used. The epoxy resin, the semiconductor chip, and the wiring substrate have different linear expansion coefficients. The connected portion with the different linear expansion coefficient may fail to absorb the stress, and if the stress is not absorbed, a crack is possibly formed in that connection portion. In order to suppress the crack, filler with a relatively small linear expansion coefficient, such as silicon dioxide, is dispersed in the under-filling agent.
Incidentally, in recent years, metal pillar bumps have been used in the flip-chip mounting. For example, a copper pillar bump has the following advantages over the conventional solder bump: the pitch of the bump can be reduced by the use of the copper pillar, the influence on the environment is suppressed because solder is used less; the copper pillar bump has such high thermal conductivity that the heat dissipation is excellent; and the electrical conductivity is so high that the parasitic resistance can be reduced. However, using the metal pillar bump may cause the filler in the under-filling agent to separate. It is considered that the separation of the filler occurs because the potential difference between the metal used in the pillar and the metal used in the solder causes electrophoresis of the filler in the under-filling agent.
In view of this, examinations have been conducted on the technique to suppress the separation of the filler from the under-filling agent (promote the dispersion of filler). Patent Literature 1 has disclosed the method of mounting the electronic component, in which covering the surface of the metal pillar with solder promotes the dispersion of filler in the under-filling and improves the connection reliability of the flip-chip mounting.